A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium, in particular to a perpendicular magnetic recording medium that can attain high density magnetic recording.
B. Description of the Related Art
In order to achieve higher recording density, magnetic recording media used in hard disk drives (HDDs) are recently being switched over rapidly from the conventional longitudinal recording system to the perpendicular magnetic recording system. The perpendicular magnetic recording system are advantageous in that the recording bits recorded on the perpendicular magnetic recording medium have more stable remnant magnetization at a higher recording density owing to the effect of a demagnetizing field from adjacent recording bits. As a result, a perpendicular magnetic recording medium exhibits excellent resistance against thermal fluctuation.
A perpendicular magnetic recording medium is provided with a soft magnetic underlayer composed of a soft magnetic material between a substrate and a recording layer. The soft magnetic underlayer sharply draws in the magnetic field generated by a magnetic head and decreases gradient of the magnetic field, to reduce influence of diffused writing signal.
For attaining higher recording density in a perpendicular magnetic recording medium, which exhibits superiorities over a conventional longitudinal magnetic recording medium, the perpendicular magnetic recording medium needs fine magnetic crystal grains, isolation between the grains, and reduction in orientation dispersion Δθ50 of the c-axis, the axis of easy magnetization of the magnetic layer. Effective means directed to satisfying these conditions have been disclosed as follows.
T. Oikawa, et al., IEEE Transactions on Magnetics, vol. 38, no. 5. September 2002 discloses a technology to form a granular magnetic layer on a ruthenium intermediate layer.
Japanese Unexamined Patent Application Publication No. 2008-034060 discloses reduction of orientation dispersion Δθ50 by an orientation control layer of a nonmagnetic material mainly composed of NiCr or NiCu under a ruthenium intermediate layer.
Japanese Unexamined Patent Application Publication No. 2002-358617 discloses improvement in orientation in a magnetic recording layer by using an intermediate layer of NiFeCr under the magnetic recording layer.
Japanese Unexamined Patent Application Publication No. 2008-084413 discloses use of another material, FeCoB, in a seed layer.
Japanese Unexamined Patent Application Publication No. 2004-310944 discloses a perpendicular magnetic recording medium comprising a substrate and a recording lamination body formed on the substrate. The recording lamination body includes a plurality of magnetic layers and nonmagnetic layers that are alternately laminated in a repeated construction by epitaxial growth from each other and have lattice constants different from each other of corresponding crystal axes in a surface perpendicular to the direction of the epitaxial growth. The medium disclosed in Japanese Unexamined Patent Application Publication No. 2004-310944 further comprises a soft magnetic underlayer, seed layers formed on the soft magnetic underlayer, and a nonmagnetic intermediate layer formed on the seed layers. The recording lamination body is formed by epitaxial growth over the seed layers through the nonmagnetic intermediate layer The seed layers consists of a nonmagnetic seed layer and a crystalline soft magnetic seed layer formed on the nonmagnetic seed layer, wherein the crystalline soft magnetic seed layer has an fcc structure and a direction of epitaxial growth of [111] direction.
Japanese Unexamined Patent Application Publication No. 2005-196898 discloses a perpendicular magnetic recording medium comprising a nonmagnetic substrate; and at least an underlayer, a magnetic recording layer, and a coating layer successively laminated on the substrate. The magnetic recording layer is composed of a lamination of magnetic layers of different materials. A magnetic layer in the underlayer side (a granular magnetic layer) comprises ferromagnetic crystal grains and nonmagnetic grain boundaries composed of oxides or nitrides. A magnetic layer in the coating layer side (non-granular magnetic layer) comprises ferromagnetic crystal grains and nonmagnetic grain boundaries with a concentration of non-ferromagnetic component set at a higher concentration than that of the same non-ferromagnetic component as non-ferromagnetic component in the ferromagnetic crystal grains. The magnetic layer in the coating layer side is formed after heat treatment on the nonmagnetic substrate having the magnetic layer in the underlayer side formed thereon. Japanese Unexamined Patent Application Publication No. 2005-196898 discloses such a medium that is further provided with a seed layer beneath the underlayer.
As described above, many techniques have been proposed for enhancing recording density of perpendicular magnetic recording media. However, performance of a perpendicular magnetic recording medium depends on the items including the component and composition of each of the laminated layers and the sequence of lamination. The techniques proposed thus far are not favorable with respect to all of the items, resulting in both merits on the one item and demerits on the other item. Therefore, further improvements in performance are required of the perpendicular magnetic recording media today.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.